Nutrient-allelochemical interactions: metabolic effects on a generalist insect herbivore

Secondary metabolites exhibit the potential to direct food selection by herbivorous insects. In this study, I examine and quantify the interactive effects on a generalist insect herbivore of a plant allelochemical and dietary macronutrients. The grasshopper Melanoplus differentialis (Orthoptera : Acrididae) was reared across the fifth larval stadium on one of 12 foods with fixed total macronutrient content (42%), but varying in protein:carbohydrate (P:C) ratio (14:28, 21:21, 28:14) and gramine content (0, 0.5, 1, or 2% dry weight). The effects of the allelochemical on consumption, nutrient uptake, development rate, and survivorship were measured. Additionally, I recorded the effects of gramine on metabolic rates a week after the beginning of the experiment, using flow through respirometry. The effects of the allelochemical on food consumption and nutrient uptake were dependent on the balance of macronutrients in the food. Calculation of the respiratory quotient (RQ) also showed interacting effect of plant allelochemical and macronutrient content on the subtrate used by the insect during respiration. I interpret these results in the context of plant secondary metabolite detoxification and discuss their significance for the field of nutritional ecology.